CD8 binds Class I MHC molecules-in this way CD8 can act as a coreceptor with the T cell receptor (TCR), for recognition of specific peptide/MHC ligands. However, CD8 also acts as an adhesion receptor for Class I MHC ligands without involvement of the TCR (this is called non-cognate Class I MHC binding). Our preliminary data show that the CD8 mediated non-cognate Class I binding changes radically during I cell maturation. This has profound implications for Cd8-mediated regulation of cytotoxic I cell (CTL) differentiation and immune reactivity. In this proposal, we will explore how CD6 mediated non-cognate Class I binding is regulated both in development and in CTL activation. We also study the nature of non-cognate Class I binding, in particular the role of the CD8beta chain. In Aim 1, we will test how changes in T cell glycosylation impacts on non-cognate Class I interactions. This builds on our preliminary data suggesting that the extent of cell surface sialylation alters the capacity of CD8 molecules to bind Class I ligands. In addition, we explore the role of ICR expression levels and a form of CD8 (called CD8 alpha prime) expressed only in immature thymocytes. In Aim 2, we study regulation of non-cognate CD8-Class I binding during CTL activation, in part to determine whether it is regulated by the same factors which control CD8 binding in development In addition, we explore the functional significance of non-cognate Class I binding on CTL cytolytic activity, a measure of the functional relevance of this binding. Finally in Aim 3, we focus on the role of CD8beta, the second chain of the CD8 molecule. We will determine whether CD8beta plays a unique role in non-cognate Class I binding and in affecting the specific contact sites on the Class I MHC molecule to which CD8 binds.